Among all known materials, metals are very widely used because of certain advantageous properties: a high level of mechanical resistance, a high level of toughness, electrical conductivity, etc. Moreover, the characteristic brilliance or lustre of such metals is very much sought after for decorative applications such as for making watch cases or suchlike. The hardness of solid gold or gold coloured coatings is however inferior to that of ceramics, which may result in significant wear in numerous common conditions of use, for example when these materials are used to make exterior parts used in watchmaking, such as watch cases or wristbands.
In order to overcome these drawbacks, it has long been sought to increase the hardness of articles made of solid gold or including a gold coloured coating. However, conventional metallurgic methods do not allow very high levels of hardness to be attained, i.e., levels of hardness greater than 1,000 HV (Vickers hardness), which would allow scratchproof decorative parts having a gold-plated metal appearance to be made.
This is why one has proposed increasing the hardness of metals by adding very hard particles. Two-phase composites have thus been made formed of a metallic phase coating hard particles, which frequently take up most of the volume of the material. A certain number of materials of this type having a gold-plated metal appearance have been developed and are today commonly used in industry. They are generally called "cermet" when they are made of a ceramic material, for example titanium nitride.
All these materials have in common however the fact that they include a distinct metallic phase which may limit their resistance to corrosion and their mechanical properties at high temperatures.
Moreover, the metallic phase used as binding agent always contains nickel or cobalt, all attempts to substitute other metals for these elements having failed until now. However, the use of nickel and cobalt, in applications which result in prolonged contact with the skin, such as, for example, for watchmaking and jewellery applications, must be avoided since these elements are known to frequently cause allergies.
Finally, these materials are generally difficult to shape. This constitutes a significant limitation for the manufacture of exterior parts for watchmaking or pieces of jewellery which often have complex shapes with resultant high manufacturing costs.